At cruise altitude the air outside is too thin to breathe, yet every passenger inhales freely without an oxygen mask. The oxygen originates from outside the aircraft, but it must travel through several stages before it reaches the passengers. The most common source of compressed air for pressurization is bleed air from the compressor stage of a gas turbine engine. This air enters the plane's pneumatic system through its engine compressors and gets directed into the primary heat exchanger. From there the air is cooled, humidified, and mixed with recirculated air by one or more environmental control systems before it is distributed to the cabin; the resulting mixture is called conditioned air.
Pressurization systems are designed to continually replace interior air with air from outside the plane, while valves on the fuselage introduce fresh air into the cabin if needed, and a spring-loaded negative pressure relief valve allows air into the cabin to even out the pressure differential, assuring that the atmosphere remains comfortable and safe throughout the flight.
Why are airplane cabins filled with pressurized air and oxygen?
Airplane cabins are filled with pressurized air and oxygen because cabin pressurization is vital for flying at high altitudes as it creates the necessary pressure for comfortable breathing during flight. At 9,144 m (30,000 ft) atmospheric pressure is only 2 kg (4.4 lbs) compared to 6.67 kg (14.7 lbs) at sea level, so the aircraft's cabin pressurization system helps create pressure equivalent to that found at 1,829 to 2,438 m (6,000 to 8,000 feet) above sea level. This maintained cabin pressure prevents hypoxia, altitude sickness, decompression sickness, and barotrauma, safeguarding passengers' and crew members' well-being. Without pressurization, a person loses consciousness in less than a minute at 30,000 ft (9,144 m), and at 40,000 ft (12,192 m) the time of useful consciousness is just a few seconds. The pressurization system continually replaces interior air with conditioned bleed air that comes from the engine compressor stage, while outflow valves regulate how quickly air is released from the cabin to keep the pressure hull - the sealed section of the airplane - at a comfortable level.
How do airplane cabins get oxygen?
Airplanes get fresh oxygen from the air outside the fuselage. The air is supplied from the compressor stage of turbine engines. This air, called bleed air, is tapped from the engines because the engines do not need all the air for combustion. Bleed air is passed through a set of machines, including the air-conditioning system, to cool it. The processed air is ultimately piped into the cabin for passengers, where it is introduced through the aircraft pressurisation system into the pressure hull.
To maintain optimal cabin conditions, an equal amount of fresh air is usually mixed with recirculated air. Airliners recirculate a percentage of the air while guaranteeing the air is refreshed every few minutes to prevent carbon dioxide buildup. The outflow valve, usually located at the rear of the airplane, collects and discharges the air inside the cabin, opening and closing in stages to regulate pressure. High-pressure air is released out of the outflow valve, assuring that the ‘used' air is vented out of the airplane and maintaining proper cabin pressure.
How is oxygen supplied in an aircraft cabin?
Oxygen is supplied in an aircraft cabin through a control system. Airplanes do not pump extra oxygen into the cabin under normal flight. Instead, the environmental control system compresses bleed air taken from the engines, cools it with the air-conditioning packs, and sends this conditioned air into the cabin, raising the pressure so that occupants breathe freely at altitudes up to about 8,000 ft (2,438.4 m) cabin equivalent. When a decompression occurs, emergency oxygen is staged: the masks drop from overhead compartments and the oxygen supply comes either from a chemical oxygen generator or from a centralized oxygen bottle system. Passengers pull a mask down and this action tugs a lanyard that triggers the chemical oxygen generator or opens a valve from the centralized oxygen bottle system. The mask allows a mixture of 100% oxygen and cabin air into the mask through an external reservoir bag and a series of one-way valves. Oxygen flows continuously into the reservoir bag while the user exhales and is inhaled during the next breath, providing breathable air until the aircraft descends to a lower altitude or the ten-minute chemical supply is spent. Crew oxygen masks, always fitted to the user's face with a minimum of leakage, draw oxygen from separate pressurised gas cylinders and can supply either diluter-demand or 100% oxygen on demand, assuring that pilots remain conscious to control the descent.
What is the percentage of oxygen in the airplane cabin?
The air in an airplane cabin contains about 21 percent oxygen at cruise altitude which remains constant while only the partial pressure changes. At sea level the partial pressure of oxygen is 159 mmHg (21.7 kPa), while at a maximum cabin altitude of 8,000 feet (2,438 meters) the partial pressure of oxygen falls to about 118 mmHg (16.1 kPa), roughly 74 percent of the sea-level value. Because the cabin is pressurized to the equivalent of 6,000 or 8,000 feet (1,828.8 or 2,438.4 meters), the oxygen concentration stays near 20.9 percent, the typical percent of oxygen in breathable air.
Is supplemental oxygen required in an airplane cabin?
Yes, supplemental oxygen is required in an airplane cabin. Cabin pressure altitudes above 12,500 feet (MSL) up to and including 14,000 feet (MSL) require the required minimum flight crew to use supplemental oxygen for that part of the flight that is of more than 30 minutes duration. At cabin pressure altitudes above 14,000 feet (MSL) the required minimum flight crew must be provided with and use supplemental oxygen during the entire flight time at those altitudes. Pressurized aircraft operated at altitudes above 35,000 feet (10,668 meters) MSL must have at least one pilot at the controls wearing a secured and sealed oxygen mask.
Supplemental oxygen works by increasing the content of oxygen going directly into the lungs to increase the partial pressure sufficiently enough to counteract the drop in overall air pressure, protecting passengers from the risks of hypoxia and boosting performance and safety for crew and passengers.
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
